1. Field of the Invention
The present invention generally relates to a method for forming a thin film employed in a semiconductor device, a sensor, an electronic component by thermally resolving a gas, and a thin film forming apparatus, and moreover an electronic device. More specifically, the present invention is directed to a method for forming an aluminum thin film or an aluminum-alloy thin film on a substrate surface, an apparatus of forming such a metal thin film, and an electronic device such as a semiconductor integrated circuit and opto-electronic devices employing an aluminum film or aluminum alloy film as a metalizing material.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of the conventional film forming apparatus. Reference numeral 1 is a processing chamber which can maintain an air tight condition therein. Reference numeral 3 denotes a substrate holder which is positioned within the processing chamber 1, holds a substrate 2, and adjusts a temperature of the substrate 2. A temperature adjusting mechanism 20 for adjusting the temperature of the substrate holder 3 will now be described. A heater 4 is employed to heat the substrate holder 3 by means of resistor heating (other heating methods such as radiation heating may be utilized). A thermocouple 5 is used to monitor the temperature of the substrate holder 3. A temperature sensing resistor may be employed instead of the thermocouple 5 as the temperature monitor. A measurement signal from the thermocouple 5 is input into a control circuit such as PID control, PI control, ON-OFF control and the like (not shown in detail), and input power to the heater 4 is controlled by employing a thyristor or relay so that the temperature of the substrate holder 3 is adjusted. The substrate holder 3 may be cooled, if required, t control the temperature thereof by way of the heating/cooling methods.
A predetermined gas 8 is conducted via a valve 7 from a gas supply apparatus (not shown) To uniformly supply this gas 8 into the substrate surface, a distributing plate 6 is employed which includes narrow holes such as multi-overlapped meshes for passing a large quantity of gas therethrough.
The above-described gas 8 which has been conducted into the processing chamber 1 is thermally resolved on the substrate 2, and a predetermined thin film is formed on the surface.
If the processing pressure contained within the processing chamber 1 is higher than 1 atm, the remaining reaction gas 11 is often exhausted via the valve 9 in a natural method. If the processing pressure is reduced below 1 atm, the residual reaction gas 11 is exhausted by way of the oil rotating pump or a combination of the roots vacuum pump and oil rotating pump.
When employing the conventional film forming apparatus shown in FIG. 1, an aluminum thin film is formed on the surface of the substrate 2 by utilizing, as the conducted gas, triethyl aluminum or tri-isobutyl aluminum which is hydrogen-diluted. Then, the following problems occur.
If the temperature of the substrate 2 is low, e.g., below approximately 400.degree. C., the sufficient film forming speed of the aluminum cannot be obtained. Also, since a whisker-shaped crystal and/or filament-shaped crystal are formed, no flat aluminum thin film .is formed As such an aluminum thin film containing the whisker-shaped crystal, or filament-shaped crystal is not a mirror-surfaced film, the mask alignment for forming IC patterns cannot be performed and the resistivity of the formed film becomes great.
When, on the other hand, the temperature of the substrate 2 is higher than, e.g., about 400 .degree. C., the sufficient film forming speed of the aluminum can be obtained, for instance, at 1000 .ANG./minutes or higher. Carbon or hydrocarbon originated from the ethyl radical, or isobutyl radical under the free state is mixed into the aluminum, so that the surface of the formed film is colored in brown or dark white.
There are many problems to utilize such a thin film as the thin film for IC interconnects, because the resistance value of this thin film is higher than that of the pure aluminum thin film.
If silicon is in contact with aluminum, this silicon is diffused into aluminum due to the high temperature, which may give an adverse influence to the characteristics of the electronic devices and cause large variations in the characteristics.
In general, as the metalizing materials for semiconductor devices, electronic components, and sensors, an aluminum thin film, or aluminum-alloy thin film is employed. Most of these aluminum-alloy thin films are manufactured by using the sputtering method.
While an integration degree of a semiconductor device is increased, and thus the pattern size thereof becomes narrower and narrower, the defects of the covering characteristics of the thin film manufactured in accordance with the conventional sputtering method at the step pattern, are particularly remarkable. Reliability at the step will be lowered and accordingly, there is a risk of circuit breakdowns.
M. L. Green et al. have proposed that the aluminum film is formed by the low pressure CVD (chemical vapor deposition) method by employing tri-isobutyl aluminum as a source material (described in "Thin Solid Films" vol. 114, pages 367-377, issued in 1984). According to this manufacturing method, the aluminum film having the better covering characteristics can be manufactured.
If, however, the pure aluminum is employed as the metalizing material, and the density of the current flowing through the pure aluminum film is great, aluminum atoms are transferred due to the electromigration, so that the circuit breakdowns or short circuits may occur. In addition, at the boundaries between the silicon substrate and aluminum, the aluminum penetrates into the silicon readily, which is brought into the instable boundary characteristics.
To prevent the above-described electromigration and stabilize the boundaries, it is known to employ an aluminum-silicon alloy which is made by conducting a small quantity of silicon into aluminum According to the conventional sputtering method, the aluminum-silicon alloy film can be easily obtained by employing the aluminum-silicon material as the target material. However, it is also difficult to obtain the aluminum-silicon film having the better step coverage.
Also, M. J. Cooke et al. attempted another method by which silicon is contained into the aluminum film fabricated by the low pressure CVD method (described in "Solid State Technology", December 1982, pages 62-65). In accordance with this method, after the aluminum film is manufactured by utilizing tri-isobutyl aluminum as the material gas by way of the low pressure CVD method, the temperature of the substrate having the aluminum film is increased and then, in contact with silane, so that the aluminum-silicon film could be obtained.
In accordance with the above-described film forming method proposed by M. J. Cooke et al., there is a drawback that better mass productivity to form the aluminum-silicon film cannot be expected because of two manufacturing steps.
In general, the surface flatness of the aluminum alloy film is represented by reflecting of light. The aluminum alloy film having a thickness of 0.8 to 1 .mu.m, utilized as a normal interconnecting film, manufactured by M. J. Cooke et al., owns the light reflectivity of 10 to 20%. This implies that a large quantity of concave and convex portions are formed on the surface of the resultant aluminum alloy film, and a very poor aluminum alloy film is obtained.
This reflectivity characteristic has an important role such that not only a degree of surface roughness is represented, but also the pattern alignment is enabled in the device processing step. As a consequence, it is impossible to perform the pattern alignment unless the light reflectivity exceeds over a predetermined threshold value. As this threshold value, the resultant light reflectivity must be more than 50%. Since the pattern alignment during the device processing step for the film fabricated by M. J. Cooke et al., cannot be executed, it is very difficult to introduce this conventional method into the manufacturing stage of the thin film.
Another conventional problem is present in the film forming velocity.
The film forming velocity according to the method of M. L. Green et al. is, for instance, on the order of 30 nm/min, whereas it is 200 nm/min at maximum, and normally 15.+-.5 nm/min according to M. J. Cooke et al.'s method. Both the film forming velocities are considerably lower than 500 nm/min, as the practical mass-production velocity level.
It is very internally diffusive between a silicon substrate and pure aluminum. Accordingly, even if pure aluminum is merely deposited on the silicon substrate, aluminum on the surface of the substrate diffuses into silicon, and the boundary characteristic is deteriorated. As a consequence, such a process is required that silicon is contained in the aluminum film, which is simultaneously effected with the deposition.
Most of the conventional electronic devices employ the aluminum films manufactured by the vapor deposition method or sputtering method as the interconnecting material, because the aluminum films have low resistivity and better stability. However, if the aluminum films manufactured according to the above-described method are utilized as the interconnecting material in the silicon semiconductor devices, the internal diffusion between silicon as the substrate and aluminum becomes great so that the stability at the contact portion is deteriorated (referred to as "penetration"), and in order to prevent occurrence of electromigration and stress-migration, the aluminum-silicon alloy film is employed as the interconnecting material.
It should be noted that the aluminum-silicon alloy film defined in the specification contains such a film that silicon is present (or segregated) in the grains and grain boundaries of aluminum-silicon.
FIG. 2 illustrates a schematic diagram of an N type silicon gate MOS (metal oxide semiconductor which has been known in the art).
In this figure, reference numeral 30 denotes a P type silicon substrate doped by boron. Reference numeral 12 represents a silicon oxide film formed by exposing the substrate 30 into the high temperature atmosphere so as to grow it therein. Reference numeral 13 indicates an N.sup.+ layer formed by patterning the silicon oxide film 12 and ion-injecting phosphorus ion injection into this patterned portion. Reference numeral 14 is a silicon oxide film grown on the N.sup.+ layer 13 by way of the CVD method. Reference numeral 15 represents a gate oxide film fabricated in such a way that after the silicon oxide film 14 is patterned, the patterned silicon oxide film 14 is exposed into the high temperature oxide atmosphere to grow it. Reference numeral 16 is a polysilicon gate layer grown on the gate oxide film 15. Reference numeral 17 indicates an aluminum film, or aluminum-silicon alloy film (simply referred to as an "aluminum film") fabricated over the entire substrate 30 by way of the evaporation method, sputtering method, thermal CVD method, or the like. Reference numeral 18 denotes a silicon tangsten film formed by the CVD method or sputtering method in order to reduce the surface reflectivity of the aluminum film 17. Reference numeral 19 represents a passivation film such as silicon nitride.
Normally, thickness of the aluminum film 17 fabricated by the evaporation or sputtering method as the interconnecting material for the semiconductor device having the above-described structure is 1 .mu.m approximately. However, as shown in FIG. 3, grains 22 having on the order of 1.5 .mu.m are formed. If such grains 22 are formed, the electromigration occurs from the conjunction part between the grains 22, with the result that the electronic device characteristic is varied, the circuit breakdowns may occur or the shortcircuit problems may occur. In addition, a hillock (a hill-shaped projection) may be produced on the aluminum interconnects due to the anneal required for the subsequent manufacturing process.
To avoid such conventional problems, copper is furthermore added to this aluminum-silicon alloy, and this copper is segregated on the conjunction part between the grains so as to improve the characteristic of this conjunction part, very recently. As a result, occurrences of the electromigration or stress-migration can be avoided. However, if copper is added as described above, this copper remains as the unetched remainder when the patterning of the aluminum-silicon-copper alloy film is carried out by the dry etching process.
To eliminate this copper remainder, the ion sputter etching may be also carried out. However, this ion sputter etching process may cause damages in the resist, or the characteristic of the device is varied by irradiating the high energy ion.
The growth of the aluminum film 17 by the thermal CVD is described in the following publications:
(1) [LPCVD ALUMINUM FOR VLSI PROCESSING] R. A. Levy and M. L. Green J. Electrochem. Soc. 134 (1987) P37c PA1 (2) [LPCV of Aluminum and Al-Si Alloys for Semiconductor Metallization] M. J. Cooke R. A. Heinecke R. C. Stern Solid State Technology December 1982 P62-.gamma. PA1 (3) "Aluminum film forming and crystallinity control by ICB method", Monthly Japanese magazine "Semiconductor world" 1987, March, Page 75 by Yamada and Takagi. PA1 (4) "Aluminum film formation by MPCVD", monthly Japanese magazine "Semiconductor World" 1987, March, Page 84 by Kato and Ito.
The thermal CVD methods described in the above publications employ the similar hot-wall type CVD apparatuses. That is, the substrate is aligned in the reaction chamber made of the quartz glass tube, and the process gas is flown along the axial direction of the quartz glass tube heated in the furnace from the outside of the quartz glass tube.
By employing this thermal CVD method, the aluminum film is manufactured, so that the step coverage can be improved. However, as the surface of the film fabricated becomes rough (reflectivity : about 10 to 20%), the conjunction condition between the grains is deteriorated so that the electromigration or stress-migration may occur.
The cluster ion beam vapor deposition method and magnetron plasma CVD method have been known from the below-mentioned publications so as to form the aluminum film having the better surface flatness. That is to say,
According to the method described in the above publication, the crucible containing aluminum is heated, the cluster is produced under high vacuum conditions, the cluster beam is ionized by the electron bombardment to obtain the ionized cluster beam, whereby this ionized cluster beam is irradiated onto the substrate to form the film thereon.
However, the aluminum film manufactured according to this method has the following drawback. That is, as illustrated in FIG. 4, the covering characteristic of the step coverage portion 24 is not good so that there is a risk of the circuit breakdown. As a result, this aluminum film cannot be employed as the interconnecting material for the electronic devices.
In accordance with the above-mentioned method, the N-pole and S-pole magnets are grounded on the back surface of the substrate holder maintained at the ground potential, with rotation conditions, and the RF power is supplied to the gas blow out portion positioned opposite to the substrate, so that the magnetron plasma CVD method is carried out.
As a result of this conventional method, in the formed aluminum films, carbon may be mixed on the order of several percents, and the resistivity thereof is about 4 to 10 .mu..OMEGA..cm, i.e., great. Consequently, although the inherent feature of the aluminum film interconnect is its lower resistivity (2.7 .mu..OMEGA..cm), the aluminum films fabricated by this method cannot be sufficiently utilized for this purpose because of their higher resistivity.